Non-volatile memory devices, such as flash memory devices, are often used in many electronic applications. Due to high memory densities, low power consumption, non-volatility, and scalability, flash memory has become a popular option. Flash memory devices may be used in laptop computers, cellular phones, personal digital assistants (PDAs), digital cameras, media players, etc. to store data.
Flash memory data is stored as a variable amount of charge on a floating gate that floats between a control gate and a channel. The amount of charge trapped on the floating gate changes the effective threshold voltage Vt of the memory cell. The threshold voltage Vt of the memory cell determines the state of memory cell.
Programming of adjacent memory cells could change the boundary condition for the memory cell to be programmed, and increase the threshold voltage Vt needed to be programmed properly. This phenomenon is referred to as floating gate-floating gate (Fg-Fg) interference. Fg-Fg interference is dependent on 1) the coupling ratio between the memory cell to be programmed and neighboring memory cells and 2) the change in threshold voltage Vt of the neighboring memory cells. As process scaling proceeds, the memory cells are placed closer together and the coupling coefficients between adjacent memory cells become stronger, resulting in greater Fg-Fg interference.
In multilevel cell (MLC), a greater threshold voltage distribution is necessary for storing multiple bits of data. As a result, greater change in threshold voltage distribution shifts occur which in turn increases Fg-Fg interference.